Apparatus and method for synchronizing clock modulation with power supply modulation in a spread spectrum clock system

ABSTRACT

A spread spectrum clock system ( 11 ) modulates the supply voltage for a circuit ( 10 ) in concert with the circuit clock frequency (C). The system increases the supply voltage for the circuit ( 10 ) in phase with increases in the circuit clock frequency (C). However, in the portion of the clock frequency modulation period in which the clock frequency (C) is decreasing, the system ( 11 ) also decreases the supply voltage for the circuit ( 10 ). This relationship between the circuit supply voltage and circuit clock frequency (C) may be accomplished by modulating the output ( 18 ) of a power supply ( 15 ) for the circuit ( 10 ) and applying that modulated supply voltage signal through a signal translator ( 30 ) to control modulation of a clock source ( 14 ).

TECHNICAL FIELD OF THE INVENTION

The invention relates to electronic circuits that use high frequencyclock signals. More particularly, the invention relates to an apparatusand method for modulating the supply voltage to a circuit along with theclock frequency in a spread spectrum clock arrangement.

BACKGROUND OF THE INVENTION

The high frequency clock signals used in many electronic circuitsgenerate coincidental electromagnetic fields. These coincidentalelectromagnetic fields or emissions are known as electromagneticinterference or “EMI. ”The energy of EMI emissions from a particularcircuit is directly proportional to the frequency of the clock signalused in the circuit.

EMI emissions from electronic devices are regulated by governmental orother agencies to certain maximum allowable limits. Electronic equipmentmanufacturers must reduce EMI emissions as much as possible in order tomaintain these emissions within the allowable limits. For example,electronic circuits may be enclosed in special electrically conductivehousings which block or shield EMI emissions from the enclosed circuit.However, as clock frequencies increase, complete EMI shielding becomesmore difficult, and EMI emission levels may increase.

Many regulatory EMI limits are set as a maximum average emission energylevel over an operating period for the circuit. Reducing clock frequencyin a circuit for a portion of an operating period reduces the averageenergy of EMI emissions over the operating period. Thus, it is sometimespossible to meet regulatory limits for EMI emissions from a particularcircuit by modulating the clock frequency in the circuit within acertain range about a centerline or nominal clock frequency. Thismodulated clock signal frequency in an electronic circuit is commonlyreferred to as a spread spectrum clock signal.

All processors and other electronic devices that operate under controlof a clock signal are limited in the clock frequency they can support,and thus the speed at which they can process data. This operationalspeed limit is the result of certain critical paths between functionalblocks which are clocked by a common clock signal. The maximum time ittakes to launch data from one circuit functional block, transmit it to areceiving circuit functional block, and arrive at the receiving circuitfunctional block prior to the setup time required by the receivingcircuit functional block, determines the minimum instantaneous cycletime that may be allowed for a clock signal in an electronic devicewhich includes the two functional blocks. This minimum instantaneouscycle time translates to a maximum clock frequency for the circuit andcan never be violated without running the risk that the circuit willproduce incorrect results for a given input. Where the clock frequencyfor a circuit is modulated in a spread spectrum clock arrangement, themodulated frequency must be controlled so that the instantaneousfrequency at any given time remains below the maximum allowable clockfrequency supported by the circuit.

Minimum cycle time may be improved or reduced in many circuits byincreasing the supply voltage in the circuit. Thus, the maximumallowable instantaneous frequency in a spread spectrum clock signal maybe increased simply by increasing the supply voltage to the circuit.However, increasing the supply voltage in a circuit will increase powerdissipation in the circuit, and the thermal effect of this increasedpower dissipation may have a detrimental impact on the functionality andreliability of the circuit. Thus, in many systems, increasing supplyvoltage level is not a viable choice to meet limitations on the maximuminstantaneous frequency in a spread spectrum clock system.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an apparatus and method forproviding a spread spectrum clock signal in a manner which facilitatesincreased circuit performance.

This object is accomplished by modulating the supply voltage for acircuit in concert or synchronization with the circuit clock frequency.According to the present invention, the supply voltage for a circuit isincreased as clock frequency is increased. However, in the portion ofthe clock frequency modulation cycle in which the clock frequency isdecreasing, the supply voltage for the circuit is also decreased. Thismodulation of the circuit supply voltage level in concert with thesystem clock frequency provides the reduced EMI emission levels desiredin spread spectrum clock arrangements, and simultaneously allows thecircuit to function periodically at higher clock frequencies to improveoverall system performance.

The relative modulation in circuit supply voltage level and clockfrequency according to the invention may be accomplished in a number ofdifferent fashions. In one form of the invention a modulator isoperatively connected to apply a first modulation to the supply voltagefor a circuit. A corresponding modulating arrangement uses the modulatedsupply voltage signal to control a corresponding modulation in the clockfrequency for the circuit. Alternatively, a modulation signal source maybe used directly to control both the modulation of the system clockfrequency and the circuit supply voltage level. Regardless of theparticular circuit structure used to effect the relative supply voltageand clock frequency modulation, the method of the invention includes thesteps of modulating one of the circuit supply voltage or the systemclock signal frequency at a first modulation frequency and modulatingthe other one of the supply voltage or clock frequency at acorresponding modulation frequency.

As used in this disclosure and the accompanying claims, a firstmodulation is a “corresponding modulation” with respect to a secondmodulation when the peaks of the first modulation waveform coincide atleast partially with the peaks of the second modulation waveform. Insome preferred forms of the invention, the supply voltage and clockfrequency are modulated so that the resulting two modulation waveformsare generally identical. However, the clock frequency modulation neednot coincide identically with the supply voltage modulation to provideincreased system performance and to fall within the scope of theinvention and the accompanying claims. That is, the two modulationwaveforms (instantaneous frequency and supply voltage level plotted overtime) may be unequal and still provide benefits according to theinvention.

These and other objects, advantages, and features of the invention willbe apparent from the following description of the preferred embodiments,considered along with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a spread spectrum clocksystem embodying the principles of the invention.

FIG. 2 is a representational timing diagram illustrating the relativemodulation of circuit supply voltage and clock frequency according tothe invention.

FIG. 3 is a diagrammatic representation of an alternate spread spectrumclock system embodying the principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates electronic circuit 10 utilizing a spread spectrumclock system 11 embodying the principles of the invention. Circuit 10 isshown in the figure as a processor, but may comprise any circuitutilizing a DC supply voltage signal and a spread spectrum clock signal.Although, circuit 10 may be implemented as a separate integrated circuitchip which receives a DC supply voltage signal and the system clock fromoff-chip sources, the circuit may alternatively be implemented togetherwith the spread spectrum clock system on a single integrated circuitchip. Also, circuit 10 and spread spectrum clock system 11 may beimplemented using discrete electronic components within the scope of thefollowing claims.

Spread spectrum clock system 11 includes a spread spectrum clock source14 and a power supply 15. Clock source 14 provides the clock signal forcircuit 10, while power supply 15 provides the supply voltage signalV_(dd) for the circuit. As used in this disclosure and the accompanyingclaims, “supply voltage signal” refers to the voltage signal supplied toand distributed throughout circuit 10 to provide the electrical energyrequired to operate the various components of the circuit. Also, “clocksignal” refers to the signal comprising a suitable clock waveform whichis supplied to circuit 10 and distributed throughout the circuit toclock or coordinate the operation of various components in the circuit.

Power supply 15 is illustrated in FIG. 1 as a operational amplifier. Thenon-inverting or first input 16 to the operational amplifier isconnected to receive a DC reference voltage signal modulated throughmodulator 17. Modulator 17 may comprise any suitable modulatingarrangement including a saw-tooth or digital waveform generator. Thepower supply output at node 18 carries the supply voltage signal V_(dd)which is applied to circuit 10. This signal is also fed back to theinverting input 19 of the operational amplifier.

Although the power supply 15 is shown as an operational amplifier inFIG. 1, it will be appreciated that the invention may be implementedusing any suitable power supply arrangement. A suitable power supply forpurposes of this invention and the accompanying claims comprises anypower supply in which the output supply voltage may be modulated. Evenin the operational amplifier arrangement shown in FIG. 1, filters andother signal conditioning arrangements may be included with the powersupply. These additional components which may be included in the powersupply 15 are omitted from the figures so as not to obscure theinvention in unnecessary detail.

Spread spectrum clock source 14 shown in FIG. 1 comprises a phase lockloop (PLL) arrangement including a phase detector 21, loop filter 22,and voltage controlled oscillator (VCO) 23. A divider may be included inthe feedback path of the PLL circuit, but is omitted from FIG. 1 inorder to simplify the drawing. A summing method 27 is also included inthe PLL clock source in this preferred form of the invention. The PLLclock source receives an oscillator or base frequency input B and amodulation input at 28. Base frequency B is applied as one input tophase detector 21 and provides a reference to which the clock sourceoutput signal at node 26 may be locked. The signal at modulation input28 is summed as indicated by summing method 27 to produce a modulatedsignal at an input 25 to VCO 23. This modulated signal modulates thefrequency of the clock signal output 26 so that the clock frequencyvaries within a certain range about a center or nominal frequency. Itshould be noted that if modulation input 28 to summing method 27 iszero, then the signal at VCO input 25 is identical to that of the output29 from loop filter 22, as would be the case for a conventional PLLclock source.

A PLL clock source is shown in FIG. 1 only for purposes of example. Itwill be appreciated that the invention is not limited to this particulartype of clock source. The invention encompasses any clock source inwhich the output clock signal may be modulated within a desiredfrequency range.

In the preferred form of the invention illustrated in FIG. 1, themodulated supply voltage signal at power supply output node 18 isapplied to control the modulation of the clock frequency. Since thesupply signal will normally be different from the voltage level requiredas the modulation input 28 to clock source 14, FIG. 1 includes a signaltranslator 30. Signal translator 30 receives the supply voltage signalfrom power supply output node 18 and translates that signal to form asuitable modulation input 28, that is, a modulation signal suitable forsumming method 27 to combine with output 29 from loop filter 22. In thepreferred form of the invention illustrated in FIG. 1, summing method 27may be effected by a combination of the signal translator 30 and loopfilter 22 rather than as a separate summing device. Any suitable summingarrangement may be employed to effect summing method 27 within the scopeof the present invention.

The operation of the spread spectrum clock system 11 shown in FIG. 1 andthe method of the invention may be described with reference to FIG. 1and the timing diagram shown in FIG. 2. The method of the inventionincludes modulating both the supply voltage signal and the clock signalfrequency (at 18 and 26, respectively, in FIG. 1) in concert orsynchronization with each other at corresponding modulation frequencies.In the form of invention shown in FIG. 1, the step of modulating thesupply voltage is accomplished using modulator 17 to modulate thereference voltage signal to the non-inverting input 16. This producesthe modulated supply voltage signal V_(dd) shown in FIG. 2.

The step of modulating the frequency of the clock source output at 26 inFIG. 1 is accomplished using the modulated supply voltage signal. Moreparticularly, the supply voltage signal at node 18 is conditionedthrough signal translator 30 and then summed with output 29 from loopfilter 22 by summing method 27. This summing function modulates the VCOinput signal 25 to effect a modulation of the clock source output 26.This modulation based on the modulated supply voltage signal producesthe modulated clock frequency shown at C in FIG. 2.

FIG. 2 illustrates that the modulation of the supply voltage signalV_(dd) according to the invention corresponds with the modulated clockfrequency. When the supply voltage signal V_(dd) is at its highestlevel, the clock frequency C is also at its highest level. On the otherhand, when the supply voltage V_(dd) is lowest, the clock frequency C isalso at its lowest level. This relationship between the modulated supplyvoltage V_(dd) and modulated clock frequency C produces severalbenefits. First, the modulated clock frequency causes circuit 10 toproduce a lower average EMI emission energy over a given operatingperiod. Second, the higher performance exhibited by circuit 10 at thehigher supply voltage will support the higher clock frequency at theappropriate time in the spread spectrum cycle, and the lower voltagesupplied during the slower clock frequency will help keep the powerdissipation in the circuit down to acceptable levels.

For example, assume a 2.5% clock frequency spread spectrum modulation isdesired in order to reduce EMI emissions for a particular circuit orsystem. In this case, a supply voltage modulation peak of only 2.1% overthe nominal (center) supply voltage level will support the sameperformance level as a system without the spread spectrum clock and withthe nominal supply voltage. This 2.1% supply voltage modulation peakrepresents half of the 2.5% peak-to-peak frequency modulation divided bythe approximately 0.6% change in the maximum supportable clock frequencyper % change in the supply voltage V_(dd). The increased performancelevel in this example is achieved at less than a 0.3% increase in powerdissipation. The semiconductor device junction added temperature rise inthe circuit would be less than 0.2 degrees Celsius.

FIG. 3 shows another preferred spread spectrum clock system 35 accordingto the invention. System 35 includes the same power supply 15 and clocksource 14 to provide the supply voltage and system clock, respectively,to circuit 10. However, in this alternate form of the invention themodulation input 28 to clock source 14 is provided directly from amodulation signal source 37. The output from modulation signal source 37is summed with the reference voltage V_(ref) at summing junction 38 andapplied to the non-inverting input 16 of the operational amplifiermaking up power supply 15.

It will be noted from FIG. 1 that modulator 17 cooperates with powersupply 15 shown in FIG. 1 to provide an arrangement for modulating thesupply voltage. The supply voltage signal V_(dd) and signal translator30 make up a corresponding modulation arrangement in FIG. 1 formodulating the spread spectrum clock frequency. In contrast, modulationsignal source 37 in FIG. 3 operates as the modulating arrangement formodulating the clock frequency from clock source 14, while summingarrangement 38 cooperates with power supply 15 to provide acorresponding modulating arrangement to modulate the supply voltage.

It will be appreciated by those skilled in the art that the invention isnot limited to the triangular supply voltage signal waveform illustratedin FIG. 2. In other forms of the invention the waveform produced by themodulated supply voltage signal may comprise a more complex waveform. Itshould again be noted that the modulation waveform of the supply voltagesignal and the modulation waveform of the clock frequency need not beidentical within the scope of the present invention and the followingclaims. Rather, the modulation waveforms may be somewhat different. Insome cases the waveform differences between the supply voltagemodulation and the clock frequency modulation may improve powerdissipation without loss of performance.

Many different arrangements may be employed to modulate the supplyvoltage signal in concert with the modulated spread spectrum clockfrequency. The arrangements shown in FIGS. 1 and 3 are simply preferredembodiments for accomplishing the desired relative modulation betweenthe supply voltage signal and clock frequency. Alternatively to thearrangements shown in FIGS. 1 and 3, a function generator may beassociated with the power supply 15 and the function applied to controlboth the modulation of the supply voltage signal and the spread spectrumclock frequency. The desired relative modulation may also beaccomplished with a passive distribution and decoupling networkassociated with circuit 10 which has a slightly negative impedance atthe fundamental frequency of the spread spectrum modulation. Theselatter modulation arrangements are to be considered equivalent to thoseillustrated in the figures.

Furthermore, those skilled in the art will appreciate that the inventionis not limited to the method of modulation or modulation introductioninto clock source 14 or power supply 15 shown for purposes of example inFIGS. 1 and 3. In other forms of the invention, the spread spectrumfrequency modulation could be accomplished before loop filter 22, andcould take signal forms other than voltages. Such alternative signalforms include currents and digital representations translatable as phaseerror for the clock source and voltage for the power supply. In otherforms of the invention, the modulation could be introduced into clocksource 14, and a frequency-to-voltage converter at the output of clocksource 14 could provide the modulation source for power supply 15. Anysuitable modulation technique for producing corresponding modulation inthe clock frequency and power supply voltage is to be consideredequivalent to the techniques and arrangements specifically described inthis disclosure.

The above described preferred embodiments are intended to illustrate theprinciples of the invention, but not to limit the scope of theinvention. Various other embodiments and modifications to thesepreferred embodiments may be made by those skilled in the art withoutdeparting from the scope of the following claims.

1. An apparatus for controlling the system supply voltage in a systemutilizing a spread spectrum clock signal, the apparatus including: (a)spread spectrum clock source having a modulation input; (b) a modulatingarrangement including a modulation signal source having an outputconnected to the modulation input of the spread spectrum clock source,the modulating arrangement applying a first modulation to a clock signalfrequency for the system, the first modulation varying the clock signalfrequency about a nominal value for the clock signal frequency; (c) acorresponding modulating arrangement operatively connected to apply acorresponding modulation to a system supply voltage the correspondingmodulation varying the system supply voltage about a nominal value forthe system supply voltage.
 2. The apparatus of claim 1 further includinga power supply circuit having a reference input, and wherein themodulation signal source output is applied to modulate a signal at thereference input.
 3. The apparatus of claim 2 further including a summingjunction connected to sum a DC reference voltage and the modulationsignal source output to produce a summed output and apply the summedoutput to the reference input of the power supply circuit.
 4. Theapparatus of claim 1 wherein the first modulation and the correspondingmodulation comprise unequal waveforms.
 5. A spread spectrum clock systemincluding: (a) a spread spectrum clock source having a frequencymodulation input and providing a clock signal; (b) a power supplycircuit providing a supply voltage output; (c) a modulating arrangementincluding modulator connected to provide a modulate reference input tothe power supply circuit, the modulating arrangement applying a firstmodulation to the supply voltage output, the first modulation varyingthe supply voltage output about a nominal value for the supply voltageoutput; and (d) a corresponding modulating arrangement operativelyconnected to apply the supply voltage output to produce a correspondingmodulation in the frequency of the clock signal, the correspondingmodulation varying the frequency of the clock signal about a nominalvalue for the frequency of the clock signal.
 6. The apparatus of claim 5further including: (a) a signal translator connected to receive thesupply voltage output and provide a translated output to the frequencymodulation input of the spread spectrum clock source.
 7. The apparatusof claim 5 wherein the first modulation waveform and the correspondingmodulation waveform are unequal.
 8. A method for providing a spreadspectrum clock signal for a circuit, the method including the steps of:(a) modulating a power supply signal for the circuit at a firstmodulation to vary the power supply signal about a nominal supplyvoltage; (b) conditioning the modulated power supply signal for thecircuit to produce a conditioned signal at the first modulation; (c)applying the conditioned signal to a modulation input of a spreadspectrum clock source circuit to modulate the frequency of the clocksignal for the circuit at a corresponding modulation to vary thefrequency of the clock signal about a nominal clock signal frequency. 9.The method of claim 8 wherein the step of modulating the power supplysignal for the circuit includes the step of: (a) modulating a referencevoltage input to a power supply for the circuit.
 10. The method of claim8 wherein the first modulation waveform and the corresponding modulationwaveform are unequal.